1. Field of the Invention
In phased array antennas, multiple signals (such as radar, electronic warfare (EW), and communications waveforms) can each be simultaneously transmitted only through a different sub-aperture of the array. For maximum power and efficiency, the power amplifiers operate on one signal at a time. This invention is to form multiple signals as a common waveform and transmit them through a common shared array aperture. The user signals are thus transmitted simultaneously and independently with full antenna gain in any direction.
2. Description of the Related Prior Art
The number of electronic equipments and associated antennas carried on military platforms continues to grow rapidly. In many instances, the platforms can no longer properly carry, nor operate, all the desired electronic equipment. To alleviate this problem, a significant initiative called the Advanced Multi-Function Radio-Frequency Aperture Concept (AMRFC) was undertaken by the Office of Naval Research (ONR) and Naval Research Laboratory (NRL), and is discussed in P. K. Hughes and J. Y. Choe, “Overview of Advanced Multifunction RF System (AMRFS),” Proceedings of the 2000 IEEE International Conference on Phased Array Systems and Technology, May 2000. The AMRFC objective is to provide many military electronic services for communications, radar, and electronic warfare (EW) by means of shared electronic equipment and through the use of a common antenna.
Currently, if multiple signals, such as radar and EW, are to be simultaneously transmitted in time, each individual signal is transmitted through a different sub-aperture of the antenna such that any power amplifier in the array only operates on one signal at a time. The sub-aperture that transmits each signal is dynamically allocated. It is necessary to transmit one signal at a time through a power amplifier since the power amplifiers operate at saturation for maximum power and efficiency. If two signals are simultaneously present at the power amplifier input, the resulting output signal will generally be highly distorted and contain extremely high intermodulations, thus causing serious problems for the nearby receiving antenna. Alternately, if the amplifier is operated in a linear mode, significant power and efficiency are lost. Consequently, at the present time only one type of signal at a time is distributed to each sub-aperture. As a result, the full gain of the antenna cannot be realized on any of the user signals that are required for simultaneous transmission.
Clearly, to form and transmit a noise-like combined waveform without mutual interaction, the signals' spectra must be well confined and located anywhere within the available frequency band, as long as they do not overlap. However, sharply confining the spectra requires signal shaping in the time domain that causes amplitude modulation of the waveforms, as discussed in W. M. Waters and B. R. Jarrett, “Bandpass Signal Sampling and Coherent Detection,” NRL Report 8520, December 1981. It is desirable to pass these combined spectrally clean, amplitude-varying waveforms through conventional power amplifiers that are operated near their full-rated RF power levels for greater efficiency. Since the power amplifiers exhibit nonlinear operation, the combined amplitude-varying input signals need to be converted into signals of constant amplitude before being transformed back to a diverse high power signal.
Here a new technique along with its practical implementation is invented, which provides a transformation on the sum of multiple signals through the Chirex outphasing, as discussed in Chirex, H., “High Power Outphasing Modulation,” Proc. IRE, Vol. 23, No. 11, November 1935, prior to power amplification, and an inverse transformation of the combined signal through power amplification. This technique allows simultaneous transmission of multiple signals through every saturated power amplifier in the array antenna without serious intermodulations and spectral distortion. In the case that these multiple signals are radar and EW waveforms, the radar waveform cannot degrade the effectiveness of the Electronic Attack (EA) technique, and similarly the EA technique cannot interfere with the radar's target-detection function. Since the individual signals of this combined waveform are spectrally confined and can be hopped about the common portion of the available frequency bands, all Navy platforms such as ships and decoys could use this waveform to prevent Electronics Surveillance (ES) systems from distinguishing between them. Transmission of this diverse waveform can deny antiradiation missiles (ARMs) from acquiring and targeting Navy platforms. On the other hand, Navy ships can target hostile platforms without being targeted themselves.